What the signal represents
The lamp can ionise molecules when its energy is sufficient for their ionisation potential. The instrument converts the resulting signal into a displayed reading. Many volatile organic compounds respond, but not every gas does, and sensitivity varies between compounds. A display marked in ppm does not mean the instrument is selectively measuring the particular substance of concern or distinguishing it from other detectable vapours in the area.
Lamp, calibration and response factor
Check compatibility between the target contaminant, lamp energy and instrument range. Calibration often uses a reference gas, with response factors applied for other substances. These factors depend on the detector and conditions documented by the manufacturer. Selecting a chemical name in a menu changes the interpretation of the signal; it does not make a PID selective when an unknown mixture is present. The calibration basis should accompany reported results.
Where the information is useful
A planned survey can show where vapour levels rise or when a task produces an emission. This information helps locate leaks and choose times for occupational hygiene sampling. For example, a rise when a container is opened supports investigation of that operation. It does not by itself establish vapour identity, an individual’s exposure over a complete shift, or compliance with a substance-specific limit.
Factors that may affect readings
Humidity, contamination of the lamp window, operating range and responses to other compounds can affect results. Response time and the sampling arrangement also require consideration. Follow the manufacturer’s checks and maintenance instructions and retain calibration records. A zero reading for a contaminant to which the sensor does not respond is not evidence of safety. Instrument limitations should be considered before entering an area rather than inferred from an apparently reassuring display.
Integrating a PID into an assessment
Record the instrument and lamp, calibration gas, applied response factor, location, time, task and environmental conditions. Use a complementary specific method where identification or quantification of an individual compound is needed. A PID does not automatically replace oxygen or flammability sensors either. Interpretation must answer a defined question: locating an emission, observing a trend or supporting an assessment strategy, without assigning capabilities the instrument does not possess.
When investigating a peak, record the product opened and any nearby sources. Timing helps identify where to investigate but does not chemically identify a compound. A decision about one substance may need a complementary selective method, particularly where the mixture changes while the instrument continues recording the same type of displayed value.
